U.S. patent number 3,967,446 [Application Number 05/500,634] was granted by the patent office on 1976-07-06 for exhaust relief silencing apparatus for marine propulsion systems.
This patent grant is currently assigned to Brunswick Corporation. Invention is credited to Joseph H. Harralson, William F. Wurzbach.
United States Patent |
3,967,446 |
Harralson , et al. |
July 6, 1976 |
**Please see images for:
( Certificate of Correction ) ** |
Exhaust relief silencing apparatus for marine propulsion
systems
Abstract
A tuned exhaust gas relief system for marine propulsion systems,
for example an outboard motor, includes a lower drive shaft housing
coupled to a two stroke engine by a pair of intermediate stacked
exhaust extension plates. The housing directs the exhaust gas
downwardly to a through-the-hub exhaust propeller for exit
therethrough. With the unit in reverse or idling, exhaust gases are
trapped within the housing. A pair of tuned exhaust relief
passageways may be formed by cavities in the mating faces of the
two extension plates with a pair of inlet openings in the lower
wall of the bottom plate. A baffle member may overlie the inlet
openings. The passageways define constant cross-sectional area
channels which terminate in exhaust openings in the rear wall of
the drive shaft housing. A resonant cavity, which may be integrally
formed by mating recesses in the opposed faces of the extension
plates, is in close spaced relation to each idle exhaust
passageway. Connecting openings are distributed along the length of
the common wall between the cavity and passageway. An exhaust
housing within the drive shaft housing forms an exhaust expansion
chamber of high acoustical impedance between the lower ends of the
exhaust pipe means and the upper wall extension plate assembly. The
idle exhaust passageways have a low acoustical impedance, so that
the acoustical impedance mismatch between the chamber and the
passageways attenuates the high frequency sounds. The idle exhaust
passageways may transmit the half wave frequency and even
multiplies thereof. The resonator cavity is selected to attenuate
the particular half wave frequency, or an even multiple thereof,
which because of physical space requirements may be present and not
attenuated by the idle exhaust passageway means.
Inventors: |
Harralson; Joseph H. (Oshkosh,
WI), Wurzbach; William F. (Oshkosh, WI) |
Assignee: |
Brunswick Corporation (Skokie,
IL)
|
Family
ID: |
23990279 |
Appl.
No.: |
05/500,634 |
Filed: |
August 26, 1974 |
Current U.S.
Class: |
60/312; 440/89A;
440/89J; 440/89R; 440/88K; 440/89D; 440/89G |
Current CPC
Class: |
B63H
20/245 (20130101); F01N 13/12 (20130101); F02B
61/045 (20130101) |
Current International
Class: |
F01N
7/00 (20060101); F01N 7/12 (20060101); F02B
61/00 (20060101); F02B 61/04 (20060101); F02B
027/02 (); B63H 005/13 (); F01N 007/12 () |
Field of
Search: |
;60/310,312,313,314
;115/17,.5E ;181/39 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hart; Douglas
Attorney, Agent or Firm: Andrus, Sceales, Starke &
Sawall
Claims
We claim:
1. In an outboard propulsion unit including an exhaust housing
means having a main exhaust passageway means terminating in a main
exhaust nozzle means for discharging of exhaust gases, said main
exhaust nozzle means being below the idle water line whereby water
moves into the main exhaust passageway means and creates a back
pressure condition in said exhaust passageway means at idle, an
idle exhaust passageway means communicating with the main exhaust
passageway means and terminating in exhaust port means to provide
essentially unrestricted flow therefrom, said idle exhaust
passageway means including silencing means having a resonator
cavity means located adjacent to said idle exhaust passageway means
and connected thereto by at least one opening and operable to
modify the flow of exhaust gases through the idle exhaust
passageway means and limit the pressure waves therein caused by
engine operation.
2. The propulsion unit of claim 1 wherein the silencing means
includes baffle plate means located in overlying relationship to
the entrance to said idle exhaust passageway means.
3. The propulsion unit of claim 1 wherein said idle exhaust
passageway means has an essentially constant cross-section.
4. The propulsion unit of claim 3 wherein said idle exhaust
passageway means has a rectangular cross-section including a narrow
extending passageway to attenuate high frequency pressure
waves.
5. The outboard propulsion unit of claim 1 having a multiple
cylinder engine with a pair of said idle exhaust passageway means
with individual silencing means and wherein each of said silencing
means includes a resonator cavity means located adjacent to said
exhaust passageway means and connected thereto by a plurality of
restricted flow openings.
6. The outboard propulsion unit of claim 1 wherein said housing
means has an upper exhaust passageway wall assembly located above
the water line and said idle exhaust passageway means is located
within said wall assembly and includes an inlet opening to the
upper portion of said main exhaust passageway means, said silencing
means being located in said wall assembly.
7. The outboard propulsion unit of claim 6 having a tuned exhaust
system including main exhaust pipe means terminating within said
exhaust housing in downwardly spaced relation to said upper wall
assembly, the housing defining an expansion chamber of high
acoustical impedance between the lower ends of the exhaust pipe
means and the upper wall assembly, and said idle exhaust passageway
means forming a finite length low acoustical impedance passage, so
that an impedance mismatch is created in the idle exhaust system to
minimize sound power transfer.
8. The propulsion unit of claim 7 wherein said idle exhaust
passageway means has a uniform, narrow width to reduce the resonant
frequency and increase attenuation of high frequencies, said idle
exhaust passageway means having negative attenuation at half-waves
and multiples thereof, said resonator cavity means being tuned to a
low even multiple of said half-wave frequency.
9. The propulsion unit of claim 8 wherein said resonator chamber is
tuned for a resonance frequency of 1000 hertz.
10. The outboard propulsion unit of claim 6 wherein said wall
assembly includes a top and a bottom plate means mounted in
abutting relation upon the upper end of said exhaust housing means,
said top plate means being adapted to support an internal
combustion engine having an exhaust manifold means terminating in
the upper face of said top plate means, said plate means having a
centrally located exhaust extension opening means and said bottom
plate means having a depending exhaust pipe means, said idle
exhaust passageway means being located at the interface of said
plate means, and extending about the outer side of said exhaust
extension opening means, said resonator cavity means being located
adjacent the one side of said idle exhaust passageway means and
coupled thereto by a plurality of lateral opening means.
11. The outboard propulsion unit of claim 10 wherein said exhaust
manifold means and extension opening means and pipe means include a
pair of side-by-side main exhaust passageways defining a tuned
exhaust system, said idle exhaust passageway means includes a pair
of idle exhaust passageways correspondingly located to the opposite
sides of said main exhaust passageways, each of said idle exhaust
passageways including an inlet opening located in the bottom
surface of the bottom plate means and generally in alignment with
the outer side of the extension opening means, said idle exhaust
passageway extending rearwardly from said inlet opening in
close-spaced relation to said extension opening means and
terminating behind said extension opening means in alignment with
the inlet opening, a separate resonator cavity means for each idle
exhaust passageway means located immediately outwardly of the
corresponding idle exhaust passageway means, each of said cavity
means including an enlarged forward portion located adjacent the
forward portion of the idle exhaust passageway means and a narrow
portion located adjacent the portion laterally aligned with the
extension opening means, coupling openings located in the wall
adjacent the narrow portion.
12. The outboard propulsion unit of claim 6 including a lower unit
having an exhaust nozzle means and secured to the lower end of the
housing means and an internal combustion engine mounted on said
upper wall assembly, a tuned exhaust pipe means secured to the
upper wall assembly and extending downwardly through said exhaust
housing and terminating in upwardly spaced relation to said lower
unit to discharge the exhaust gases into the housing for
discharging of the exhaust gases through said nozzle means below
the water from the lowermost end of the unit, the lower end of said
exhaust pipe means extending below the idle water line of the
housing, said idle exhaust passageway means including an inlet
opening forwardly of the pipe means and extending rearwardly about
the pipe means and then downwardly through said housing and
terminating in said exhaust port means located above the idle water
line of the unit, said resonator cavity means coupled by restricted
opening means to the idle exhaust passageway means to reduce the
pulsation of the pressure waves at the exhaust ports as a result of
engine operation.
13. The outboard propulsion unit of claim 12 including a baffle
plate located in overlying downwardly spaced relation to said inlet
opening.
14. In the outboard propulsion unit of claim 1 wherein said exhaust
housing means includes a lower drive shaft housing secured to a
lower propeller unit having an exhaust nozzle concentrically of the
propeller unit for discharging of the exhaust gases through the
propeller unit, said housing means having an upper wall assembly
including a pair of plates including mating cavities forming said
idle exhaust passageway means, said cavity means located adjacent
the idle exhaust passageway means and coupled thereto by a
plurality of longitudinally spaced openings spaced in the direction
of the exhaust gas flow.
15. The outboard propulsion unit of claim 14 includes a pair of
said idle exhaust passageway means, each of said passageways having
a similar cavity means.
16. The outboard propulsion unit of claim 14 wherein said idle
exhaust passageway means is an elongated passageway and said
resonator cavity means is a parallel narrow cavity coupled to the
central portion of said idle exhaust passageway means by said
plurality of openings.
17. The apparatus of claim 16 having a pair of tuned exhaust
systems and a pair of said idle exhaust passageway means including
separate resonator cavities, each of said cavities having a volume
of approximately 1.2 cubic inches and coupled to the exhaust
passageway means by three openings of about 1/8 inch diameter.
18. The outboard propulsion unit of claim 16 wherein the silencing
means includes baffle plate means located in overlying relationship
to the entrance to said idle exhaust passageway means.
19. In a marine propulsion unit including an engine, a propelling
element, a drive train connecting said engine and propelling
element and a housing for said drive train including a main exhaust
passageway therethrough, said housing and the main exhaust outlet
therein normally extending below the water, means for relieving
exhaust pressure within said main exhaust system primarily where
said unit is at idle or in reverse comprising a channel of
preselected cross-sectional area and length venting said main
exhaust passageway to the atmosphere, the dimensions of said
channel being selected to attenuate selected audio frequencies, and
a side branch resonating cavity connected to said channel having
dimensions selected to attenuate selected audio frequencies.
20. The device of claim 18 wherein said main exhaust passageway
includes an expansion chamber and the inner end of said exhaust
pressure relieving channel is open to said chamber.
21. The device of claim 19 including a plurality of said channels
and connected cavities each of substantially identical dimensions.
Description
BACKGROUND OF THE INVENTION
This invention relates to an exhaust silencing apparatus for
outboard motors and other marine propulsion apparatus.
Outboard motors are generally constructed with an internal
combustion engine as the power head. Various exhaust systems are
employed for the engine, some of which provide tuning for maximum
power at wide open throttle conditions. One highly satisfactory
system channels the exhaust through the propeller hub. In such
constructions, exhaust pipes or passageways are projected
downwardly through the drive shaft housing and terminate in a
passage extending downwardly through the propeller unit for exhaust
below the water level. A typical system is illustrated in U.S. Pat.
No. 3,808,807. When the engine is idling, however, the lower end of
the exhaust discharge passageway is submerged within the water,
creating excessive back pressure conditions. Generally, a pressure
release system is provided by allowing the exhaust gases to escape
upwardly through the water into the drive shaft housing and then
outwardly via an idle relief exhaust passageway provided in the
upper portion of the drive shaft housing and terminating above the
idle water line. Although such systems provide a satisfactory
solution to the back pressure problem, such exhaust passageways
have been a source of undesired and considerable noise under idle
speed conditions.
SUMMARY OF THE PRESENT INVENTION
The present invention is particularly directed to a marine
propulsion apparatus having a main exhaust through the lower unit
of the apparatus in combination with an idle relief exhaust system
through the upper portion of the drive shaft housing, including
special means for minimizing noise generated as a result of the
exhaust gases passing through the idle relief exhaust system.
Generally, in accordance with the present invention, the idle
relief exhaust passageway is especially formed with sound deadening
means to reduce the noise generated as a result of passage of the
exhaust gases. In a particularly novel aspect of the present
invention, a resonant cavity or chamber is coupled to the idle
relief exhaust passageway and designed to significantly reduce the
noise level. The resonant chamber is formed immediately adjacent to
the idle exhaust passageway and coupled thereto by a plurality of
interconnected openings. The pressure sound waves flow into and
from the resonant chamber in accordance with the exhaust pressure
conditions. The chamber dimensions are such that resonance occurs
at the frequency, or even multiples thereof, at which silencing is
desired. Energy required to drive the oscillations in the resonant
chamber is lost from the sound waves with a resulting attenuation
of sound levels at those frequencies. The restricted openings
provide damping of the pressure wave. The size of the chamber and
the interconnection thereto all contribute to the sound deadening
characteristics.
The invention is particularly effective in tuned exhaust systems
wherein an exhaust pipe means terminates within an exhaust housing
chamber for transfer and discharge of the exhaust gases through the
lower end of the exhaust housing when underway. The housing chamber
is formed with the idle exhaust passageway in an upper wall
assembly. The exhaust housing chamber forms an exhaust expansion
chamber of high acoustical impedance between the lower ends of the
exhaust pipe means and the upper wall assembly. The idle exhaust
passageway in the upper wall assembly has a low acoustical
impedance, so that the resulting acoustical impedance mismatch
between the chamber and passageway attenuates the high frequency
sounds. The idle exhaust passageway may transmit the half wave
frequency and even multiples thereof. The resonator chamber is
selected to attenuate the particular half wave frequency, or an
even multiple thereof, which, because of space restrictions, may be
present and not attenuated by the tuned idle exhaust
passageway.
In accordance with a further aspect of the invention, applicants
found that a baffle means over the exhaust passageway inlets help
to minimize the exhaust noise.
Within the broadest aspects of the present invention, applicants
have also found that an apertured baffle plate located within the
idle exhaust passageway results in a reduction in the noise level,
although the adjacent resonant cavity has been found to provide
highly superior results.
In particular when applied to a multiple-cylinder, tuned engine
forming a part of an outboard motor, the drive shaft housing is
coupled to the power head with a pair of intermediate stacked
exhaust extension plates. The lower plate includes a pair of
integrally cast exhaust pipes connected by appropriate passageways
in the plates to a pair of exhaust channels from the engine. The
exhaust pipes are selected for proper tuning of the engine. A pair
of idle relief passageways are cast in the mating faces of the two
extension plates with a pair of inlet openings in the lower wall of
the bottom plate located, respectively, to the opposite sides of
the exhaust tubes. An integral baffle member is formed overlying
the inlet openings of the idle relief passageways. The top and
bottom extension plates are cast with opposed mating recesses
defining constant cross-sectional area idle exhaust passageways
which terminate in exhaust openings or ports in the rear wall of
the drive shaft housing. A resonant cavity is integrally formed by
mating recesses in the opposed faces of the top and bottom wall in
close spaced relation to the idle exhaust passageway. A plurality
of openings are formed in the wall of the lower plate, with the
openings distributed along the length of the idle exhaust
passageway in aligned relation and communicating with the resonant
cavity.
The tuned idle relief exhaust system described with the baffle and
resonant cavity in the extension plates is readily adapted to
commercial production, and significantly reduces the noise level
particularly at idle. Further, the concept can be readily
incorporated into the outboard motor construction without
interfering with the necessary aesthetic presentation and thus is
particularly adapted to practical implementation of the
invention.
BRIEF DESCRIPTION OF THE DRAWING
The drawing furnished herewith illustrates the best mode presently
contemplated by the inventors for carrying out the subject
invention in which the above advantages and features are clearly
disclosed as well as others which will be readily understood from
the following description of the embodiments shown.
In the drawing:
FIG. 1 is a side elevational view of an outboard motor constructed
in accordance with the teaching of the present invention and with
parts broken away to illustrate certain details of
construction;
FIG. 2 is a fragmentary rear elevational view of the lower unit of
the outboard motor taken generally on line 2--2 of FIG. 1;
FIG. 3 is an enlarged fragmentary vertical view taken generally on
line 3--3 of FIG. 2 and illustrates a tuned exhaust idle relief
passageway formed by cavities in the mating faces of stacked
exhaust extension plates;
FIG. 4 is a horizontal section through the outboard motor taken on
line 4--4 of FIG. 1 and showing the bottom side of the upper
exhaust extension plate;
FIG. 5 is a horizontal section taken generally on line 5--5 of FIG.
1 and showing the top side of the lower extension plate;
FIG. 6 is a vertical section taken generally on line 6--6 of FIG.
5;
FIG. 7 is a fragmentary sectional view taken generally on line 7--7
of FIG. 4 and through the stacked exhaust extension plates of FIGS.
4 and 5 to more clearly show the cross-section of the relief
passageways; and
FIG. 8 is a view taken on line 8--8 of FIG. 7.
DESCRIPTION OF ILLUSTRATED EMBODIMENTS
Referring to the drawings and particularly to FIG. 1, an outboard
motor is illustrated including an upper power head 1 having an
internal combustion engine 2 mounted within a cowl 3. The engine 2
and cowl 3 are supported upon the upper end of a drive shaft
housing 4 with an underwater lower unit 5 secured and carried by
the bottom end of the drive shaft housing. A propeller 6 is
rotatably supported by the lower unit 5 and a drive shaft 7 extends
through the forward portion of the housing 4 and unit 5 to connect
the output of the engine to the propeller drive gear means within
the lower unit 5. In the illustrated embodiment of the invention,
the engine 2 is assumed to have six cylinders divided into two
groups of 3 for exhaust purposes. A pair of tuned exhaust systems
terminating in a pair of exhaust tubes 8 and 9 extend downwardly
through the aft portion of the drive shaft housing 4. The exhaust
tubes 8 and 9 are selected of a desired length which in combination
with the exhaust passageways within the engine, not shown, provide
a tuned exhaust system for the engine; normally for a wide open
throttle condition. The exhaust gases 10, as shown in FIGS. 1 - 3,
flow downwardly from the exhaust tubes 8 and 9 through the drive
shaft housing 4 and exit through an exhaust passageway 11 formed in
the lower unit 5. Passageway 11 extends outwardly through the
propeller hub 12 and the exhaust gases discharge through a nozzle
opening 13 to the rear portion of the outboard motor under normal
operating propulsion conditions. Such systems provide for
minimizing of the back pressure under operating conditions as a
result of the high speed rotation of the propeller and contributes
to the efficient exhausting of the exhaust gases.
However, under low speed or idle conditions, the lower unit 5 is
submerged within the surrounding water 14 as shown in FIGS. 2 and
3, and the water will extend upwardly into the exhaust passageway
11 and cover the lowermost end of the exhaust tubes 8 and 9. Under
such conditions, the exhaust gases 10 and partially trapped
upstream of the exhaust tubes 8 and 9, resulting in back pressure
conditions. Generally, the exhaust gases 10 will escape upwardly
through the water 14, as shown by dotted exhaust gas lines 10, into
the drive shaft chambers 15 between the water level and the upper
end wall means 16 of the drive shaft housing 4. A pair of novel
idle relief exhaust passageways 17 and 18, which particularly
illustrate the subject matter of this invention, are provided in
the upper wall means 16 of the drive shaft housing to direct the
idle exhaust gases therefrom. Generally, the relief exhaust
passageways 17 and 18 similarly extend rearwardly through the wall
means 16 and terminate in a pair of exhaust ports 19 and 20 located
immediately above the normal water level 21 at idle conditions. In
accordance with the present invention, the exhaust passageways 17
and 18 are provided with sound damping means, to reduce and
minimize the noise created by passage of the exhaust gases to
atmosphere. Each of the special relief exhaust passageways 17 and
18, in the illustrated embodiment of the invention, is identically
constructed to the laterally opposite sides of the drive shaft
housing 4 and particularly to the opposite side of the exhaust
tubes 8 and 9, as shown in FIGS. 4 and 5. Only the passageway 17
shown to the left side of the assembly in FIG. 2 is described in
detail and the corresponding elements of the passageway 18 are
identified by corresponding primed numbers.
In the illustrated embodiment of the invention, the damping means
include a side branch resonator chamber 22 coupled to idle exhaust
passageway 17, and a sound deadening baffle means 23 interposed
across the opening 24 to the relief exhaust passageway 17. The side
resonating chamber 22 is coupled to the idle exhaust passageway 17
by a plurality of spaced openings 25 which provide a restrictive
flow of the exhaust gases 10 into and from the chamber 22. This
results in a dampening of the pressure waves and the resultant
silencing of the exhaust at idle. The baffle means 23 prevents
direct passage of the idle exhaust gases 10 and further contributes
to the damping of the pressure waves and therefore the silencing of
the usual noise.
More particularly, in the illustrated embodiment of the invention,
the upper wall means 16 includes a pair of exhaust extension plates
26 and 27 which are arranged in stacked relation between the engine
2 and the uppermost end of the drive shaft housing 4. The bottom or
lower plate 26 is constructed with the exhaust tubes 8 and 9
integrally cast therewith. The bottom and upper plates 26 and 27
having corresponding aligned openings 28 and 29 between tubes 8 and
9 and the lower end of the manifold exhaust passageways, not shown,
of the engine 2.
The engine extension plates and drive shaft housing are
interconnected by suitable means to form a pair of continuous tuned
exhaust passageways for efficient exhausting of the gases 10
through the propeller 6 when the propeller is engaged and the
engine is moving through the water.
The illustrated idle exhaust passageways 17 and 18 may be formed in
the top and bottom plates 26 and 27 by suitable mating cast
cavities in the opposite mating faces thereof, as follows.
Referring particularly to FIGS. 3 and 5, the bottom plate 26 is
generally a cast member having a bottom planar surface 30 resting
in sealing engagement upon the upper end of a correspondingly
configured top wall of the drive shaft housing 4. The exhaust
openings and tubes 8 and 9 are located in the rear central portion
of the plate 26 and extend downwardly into housing 4. The plate
member 26 is cored-out to reduce the total weight thereof and, in
accordance with the illustrated embodiment of the invention,
includes a cored-out portion adjacent the forward wall 31 of the
exhaust tube 8 to define a relatively thin upper wall portion 32 in
the upper plane of the bottom plate 26. The exhaust inlet opening
24 of the passageway 17 is formed within such wall.
In accordance with the present invention, the baffle plate 23 is
integrally cast within the cored-out portion of the lower plate 26
in downwardly spaced relation to the opening 24. The baffle 23
permits the upward flow of exhaust gases 10 around the edges of the
baffle plate into and through the exhaust inlet opening 24, as most
clearly shown in FIG. 3.
The bottom surface of the upper extension plate 27 is cored and
particularly formed with an upper passageway cavity 34 having one
end aligned with opening 24 defining the upper portion of the
exhaust passageway 17, as shown in FIGS. 3 and 4. Cavity 34 extends
rearwardly with a slight horizontal outward or lateral bend in the
central portion around the opening 28, as most clearly shown in
FIG. 4. The cavity 17 has a generally rectangular cross section at
the inlet opening 24 with a curved end wall 35 aligned with the
inlet opening to smoothly direct the rising exhaust gases
horizontally and rearwardly through the exhaust passageway 17.
Cavity 34 extends rearwardly over an intermediate top wall surface
of bottom plate 26 and terminates in superimposed relation to the
aft end of a generally rectangular cavity 36 which extends
rearwardy in the top wall of the bottom plate 26, as shown most
clearly in FIGS. 3, 4 and 5. The cavity extends rearwardly and
terminates in the rearmost portion in a downwardly directed opening
or passage 37. The drive shaft housing 4 has an aligned opening 38
terminating in the exhaust port 19. The top plate 27 is also
slightly relieved along the extent of the cavity 36 of bottom plate
26, as most clearly shown in FIG. 3. The mating plates 26 and 27
thus define an exhaust passageway 17 of an essentially constant
cross sectional area.
In accordance with the teaching of the present invention, the
resonator chamber 22 is formed by a pair of mating cavities 40 and
41, most clearly shown in FIGS. 7 and 8, in the top and bottom
plates 26 and 27 immediately adjacent to the passageway 17, and
particularly the inlet portion thereof.
The resonator chamber 22, as viewed in FIG. 4 or 5 is provided with
a relatively narrow rectangular aft portion 42 extending adjacent
to the exhaust passageway 17 and a slightly enlarged triangular
forward portion 43 located forwardly thereof. The enlarged portion
43 is located adjacent the inlet end or portion of the idle exhaust
passageway 17 which is set generally into alignment with the outer
side of the exhaust extension opening and provide adequate space
for portion 43. The portion 42 is located in lateral alignment with
the extension opening and thus is formed as a narrow chamber
extension within the limited space.
As shown in FIGS. 4, 7 and 8, the cavity 41 in the upper plate 27
has the enlarged portion 43 of a depth similar to the passageway 17
but the narrow portion 42 is only a slight depth. As shown in FIGS.
5 and 7, the cavity 43 in the bottom plate 26 has a constant depth
which is slightly deeper than the idle exhaust passageway cavity
36, and extends throughout both portions 42 and 43. The wall 44,
separating the passageway 36 from the cavity 41 is relatively thin
and is provided with the plurality of integrally cast apertures or
openings 25. For optimum results applicants have found the openings
25 should be limited in number and spaced longitudinally along the
length of the exhaust passageway 17 and the resonant cavity 22.
Under idle conditions, the exhaust gases 10 pass upwardly around
baffle 23, through the inlet opening 24 into the exhaust passageway
cavity 34 in the top plate 27 and then downwardly into the
extension cavity 36 in the lower plate 26, where the exhaust gases
are coupled by openings 25 to cavity 22.
The small openings 25 between the exhaust passageway 17 and the
resonant cavity 22 provide a resonant action and damping of the
pressure waves. The size and number of the openings between the
main passageway 17 and the resonant cavity 22, as well as the
volume of the resonant cavity 22, all contribute to creation of an
optimum silencing characteristic for any given system. Generally,
the size of the cavity 22 will be dictated by the available space
within the housing. However, the total optimum condition can be
readily determined by simple emperical construction of the cavity
with variations in the number and location of the openings.
The resonant cavity and openings ace act a side branch resonator.
The theory of operation is analogous to that of a machanical
vibration absorber. By tuning the resonator to a frequency which is
the same or an even multiple of the driving frequency (i.e. the
undesired sound frequency) the energy of the sound is used to drive
the oscillations of the resonant cavity and thus the sound pressure
level at the exit from the engine is reduced. By considering the
resonant cavity and openings as a helmholtz resonator, it is
possible to calculate the required physical dimensions for a
particular frequency.
As previously set forth, the invention is particularly effective
when utilized in tuned exhaust system such as in the illustrated
embodiment. Chamber 15 within the exhaust housing 4 forms an
exhaust gas expansion chamber of a high acoustical impedance. The
idle exhaust passageway 17 is formed as a low frequency resonator,
which, in the illustrated embodiment, particularly results from the
constant cross section and narrow width. The acoustical mismatch
between the chamber 15 and the passageway 17 results in effective
attenuation of essentially all higher frequencies, with increased
attenuation of essentially all frequencies above the resonant
frequency of the tuned idle exhaust passageway. The resonant
frequency is not as fully attenuated by the mismatch and resonant
idle exhaust passageway, but the side branch resonator chamber is
selected to essentially remove this sound source. In practice, the
first even multiple was selected to permit use of a practical
cavity size. Thus, for a six cylinder high speed outboard engine
with a tuned exhaust, the total exhaust flow length was about 13
inches and dips in the attenuation occurred at multiples of 500
Hertz (Hz). A resonant chamber for this frequency could be formed
conveniently with the adaptor or extension plates. Applicants found
that essentially similar results were obtained by tuning the side
branch resonator to the even multiple of the resonant frequency
which permitted a convenient sized chamber. Thus, for the described
example, tuning to 1,000 Hz reduced the required chamber volume to
1.2 cubic inches which was readily cast into the extension plates
26 and 27 without interference with the desired overall
configuration of the outboard.
The combination of the resonant cavity 22 and the input baffle
plate 23 provides a highly improved idle exhaust system with a very
significant reduction in the noise level.
Other damping methods might be employed in accordance with the
broad aspects of the present invention to obtain a similar or a
deadening noise reduction. However, none have been found to provide
as effective a result as the side branch resonators and the inlet
bafflers. For example, a baffle over the upper end of the exhaust
tube 45 which is suitably supported within the drive shaft housing
produces significant noise reduction. Applicants have found that
the baffle plate over the inlet openings to the idler exhaust
passageway provides essentially the same results while
significantly minimizing the manufacturing and casting processes
and procedures. Further, sound deadening is obtained by merely
introducing an apertured baffle plate directly across the direct
flow path of the idle exhaust passageway but such a system is
significantly inferior to the resonant cavity construction of the
present invention.
The present invention thus provides a practical marine engine tuned
idle relief exhaust system for minimizing exhaust noise under idle
conditions.
Various modes of carrying out the invention are contemplated as
being within the scope of the following claims, particularly
pointing out and distinctly claiming the subject matter which is
regarded as the invention.
* * * * *